The present invention relates to a video display apparatus for displaying a picture by modulating illumination light in accordance with a video signal and more particularly to, a lighting unit for controlling the luminance of illumination light in accordance with video signals, a video display apparatus provided with the lighting unit and a video display method using the same.
The display apparatus can be classified principally into a luminous display apparatus such as CRT (cathode ray tube) or plasma display panel and a non-luminous display apparatus such as liquid crystal display (also called a liquid crystal display apparatus or liquid crystal display panel) or electro-chromic display.
Available as the non-luminous display apparatus are an apparatus of the type using a reflection type light modulation device adapted to adjust the quantity of reflection light in accordance with a video signal and an apparatus of the type using a transmission type light modulation device adapted to adjust the quantity of transmission light in accordance with a video signal. Especially, a liquid crystal display apparatus using a liquid crystal display device (also called a liquid crystal display panel) as transmission type light modulation device and having a lighting unit (also called a backlight) on the back of the device is thin and light in weight and is therefore employed for various kinds of display apparatus including a monitor of computer and a television (TV).
Incidentally, when displaying a picture in the self-luminous display apparatus such as CRT, a specified pixel is selectively lit by a necessary quantity of light in accordance with a video signal. Accordingly, for a black display or a dark picture display, lighting of the pixel can be stopped or the lighting quantity can be decreased to reduce power consumption. Further, in the case of the black display, the pixel is not lit and the contrast ratio can be increased up to several of tens of thousands or more in a dark room.
On the contrary, generally in the non-luminous type display apparatus such as liquid crystal display apparatus, a backlight is, in general, lit constantly at a constant luminance level regardless of a video signal. Accordingly, the luminance of backlight normally matches with a condition for making the screen have maximum luminance and the backlight is lit at the same luminance even when a dark display is exhibited or a dark picture is displayed, with the result that unnecessary power not contributing to display is consumed. Further, in the case of the black display, part of light of the backlight leaks, leading to insufficient darkness and the contrast ratio in a dark room is about 500 to 1000 which is smaller than that of the self-luminous display apparatus such as CRT.
A liquid crystal display apparatus has hitherto been proposed which reduces power consumption and improves picture quality by controlling the ambient light (hereinafter specifically termed luminance) of the backlight.
For example, JP-A-2001-142409 discloses that a backlight panel is driven in units of plural partitive areas and the luminance of the backlight is controlled in accordance with video signals to thereby reduce power consumption.
Further, JP-A-2001-290125 discloses a technique according to which an electroluminescence (EL) panel having EL elements of three colors of red, green and blue is disposed on the back of a liquid crystal display panel and luminescence of the EL elements is controlled in accordance with video signals to thereby prevent such a degradation in picture quality as a blur or ooze of color during motion picture.
Furthermore, JP-A-2002-202767 discloses that when a picture has high luminance locally or the overall screen is required to exhibit high luminance in relation to a criterion of one picture frame, the luminance of backlight is raised but in the other case, the luminance of backlight is kept at a normal level, thereby realizing a high contrast ratio.